Laboratorio fisiologia

Lab 4. Cardiac muscle physiology
 
The heart's primary function is simply to act as a pump that provides pressure to move blood to its ultimate destination - the tissues. The control of cardiac contractility is complex and represents a balance of intrinsic (within the heart) and extrinsic (from outside the heart) factors. In the following experiment you will examine some of these intrinsic andextrinsic factors that make the heart such a unique and versatile pump.
 
Cardiac muscle differs from skeletal muscle both morphologically and functionally. Probably the most striking and fascinating feature of its contractility is that it is able to initiate its own rhythmic contractions without requiring a stimulation from outside the heart. This is due to "leaky" cell membranes, in whichcalcium and sodium ions slowly leak into the cells. This leaking causes a slow depolarization to threshold, thus firing an action potential and initiating contractions of cardiac muscle. The cells that are most "leaky" to ions and that depolarize fastest control the rate of contraction of all other cardiac cells; thus, they act as pacemakers for the rest of the heart. In the mammalian heart, thepacemaker is the sinoatrial (SA) node, a group of specialized cells near the junction of the vena cava and the right atrium. In the frog heart, the pacemaker is the sinus venosus, an enlarged region between the vena cava and the right atrium. (The mammalian SA node is believed to be an evolutionary remnant of the sinus venosus.)
Anatomy of Amphibian Heart. In this series of experiments you will use afrog heart because it functions well at room temperature and will continue to beat even when excised from the body. Mammalian hearts have the same contractile characteristics, but must be supplied with a constant flow of warm, oxygenated blood to maintain their contractility.

The frog heart differs from the mammalian heart anatomically in that they are three chambered rather than fourchambered. The pacemaker in the amphibian heart is the sinus venosus, a thin-walled sac that receives blood from the anterior and posterior caval veins and empties blood into the right atrium. The single ventricle receives blood from both atria and pumps blood out through the large artery called the truncus arteriosus (Figure 1). In contrast, the mammalian ventricle has separate left and right chambers,which prevent mixing of the venous and arterial blood.
Dissection Procedures Double pith a frog and fasten it to a frog board, ventral side up. Use scissors to make a longitudinal incision through the skin and body wall of the thoracic region to expose the heart. Note the pericardial sac surrounding the heart. Hold the pericardium with forceps and carefully cut away the sac from the heart, usingscissors. From this point on make sure that the heart is periodically moistened with from Ringer's solution.
Using forceps, gently lift the apex of the heart upward. Insert a bent insect pin or small fishhook through the tip of the ventricle, being careful not to damage the ventricle. Tie a thin thread to the hook and connect the ventricle to the transducer and adjust the tension on the ventricleuntil the recording pen is raised slightly above the baseline (Figure 2).
 
 
 
 
Experimental Procedure
1. Normal Heartbeat. Obtain a recording of the normal cardiac rhythm such as that seen in figure 3. Distinguish the atrial and ventricular contractions. The duration of systole and diastole of the ventricle can be determined. Attach a labeled portion of your record to the LaboratoryReport.

2. Refractory Period of Heart. Position the transducer to eliminate as much as possible the atrial contraction in the recording. Arrange for electrical stimulation of the ventricle by clamping the stimulating electrode so that the points touch the ventricle gently and constantly during the contraction cycle.
 
Record the ventricular contractions. Using single stimuli of 100mV and 1-msec...